Pliny the Elder [1st century CE] (Natural History, Book 10, 54): Only ducks and birds of the same kind soar up straight away, and move skyward from the start, and this even from water; and consequently they alone when they have fallen into the pits that we use for trapping wild animals get out again. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:51): The duck [anas] takes its appropriate name from its persistent swimming [natare]. Certain ones of the duck species are called ‘true’ [germanus] ducks because they are more nourishing than the others. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.9): A duck, as he says [i.e. Liber rerum], is a bird somewhat larger than a cock. The male has a green neck and head; but it has a wide beak, white, green, and black wings, and a white collar around its neck. He has red and broad feet, with which he wades in the waters. For it delights in the waters of the rivers, and can hardly live without them, especially when dry food makes it drink more. Ducks alone, as Pliny says, and those of the same kind, immediately lift themselves up into the air and with their whole bodies seek the sky. As soon as the chicks come out of the egg, they thrive with such agility that, even if their mother should die or be separated from them, they live without a nurse. At first ducks do not clearly recognize the chicks, soon they cry uncertainly and call together anxiously; and wail about the ponds and swamps where the young are drowning. Finally, when the offspring have begun to follow the mother more aggressively, they recognize their offspring and lead them out to graze. Ducks and geese and other birds clean themselves with the watery herb syderite. Sometimes the males, where there are several together, are driven to such a frenzy of lust that they kill the only female duck by competing with each other. - [Badke translation/paraphrase]